Fan



g- 8, 1967 K. D. MCMAHAN 3,334,807

FAN

Filed March 28, 1966 5 Sheets-Sheet 1 cow i S. INVENTOR. 63 A/CIYZON D MGM/9H8 BY 61 66 2 7 }/%y/M/ HTTOIPNCVS Aug. 8, 1967 K. D. MMAHAN FAN 5 Sheets-Sheet 2 Filed March 28, 1966 S flaw MM r M N D g- 3, 1967 K. D MCMAHAN 3,334,807

FAN

Filed March 28, 1966 5 Sheets-Sheet 5 N A f VOLUME CH7) i=1 17.11.

'55747/C P 6850 E 74 Tiqla INVENTOR. A/CWm/Y D MC/VM'AIY Wfl Aug. 8, 1967 K. D. MCMAHAN FAN 5 Sheets-Sheet 4 Filed March 28, 1966 5 ivammQ Qd HLI O W/ INVENTOR. 28099; d 0411,1 A/QVI'DN 0. /7c #4409 United States Patent 3,334,807 FAN Kenton D. McMahan, Rogers, Ark., assignor to Rotron Manufacturing Co., Inc., Woodstock, N.Y., a corporation of New York Filed Mar. 28, 1966, Ser. No. 537,800 14 Claims. (Cl. 230-120) This application is a continuation-in-part of my copending application, Ser. No. 433,286, filed in the United States Patent Office on Feb. 17, 1965, now abandoned; which application in turn is a continuation-in-part of application Ser. No. 290,408, filed in the United States Patent Ofiice on June 24, 1963, now abandoned.

The present invention relates to fans and more particularly to tubeaxial fans providing a combination of efiiciency, performance and versatility which heretofore has been unattainable in fans of its type.

The cooling of electronic equipment and other apparatus by means of fans mounted in or adjacent the cabinet is a conventional expedient. In selecting a suitable fan to cool a given piece of equipment, a multitude of factors must be considered before an intelligent choice of the particular type and size of fan can be made. Insofar as its cooling function is concerned, the equipment designer must choose a fan having sufficient capability to provide the necessary flow of air to maintain the proper temperature in the equipment, while working into the pressure determined by its environment. Where there are no restrictions on such parameters as fan size, weight and power consumption and where efiiciency can be ignored, the equipment designer has a relatively simple task.

However, in the majority of applications, especially in connection with electronic equipment now being produced in which package density is constantly being increased and overall size decreased, the limitations imposed by these additional design factors make fan selection much more complicated. Typically, greater package density of electronic components generates more heat in a given volume, requiring greater air flows for cooling, and also imposes impediments to air flow which tend to increase the pressure at which a fan must work to deliver the desired flow. In addition, the limited overall package size demanded by such equipment often places severe limitations on the external dimensions of the fan unit itself. Thus, the fan designer is faced with the neces sity of increasing the air delivery capacity of a fan while working into a higher pressure, all within a smaller package.

Still other considerations must be met. For example, in cooling apparatus used in office equipment such as computers and in high fidelity audio equipment, annoying noise levels are intolerable and fan noise must be kept to an absolute minimum. In many applications furthermore, either power supply or overall size restrictions dictate that the power consumption and size of the driving motor of the fan be as small as possible.

Axial flow fans have been widely used in cooling applications. One form of such fan, known as the tubeaxial fan, consists of a motor driving a propeller blade or blades, which revolve within a shroud to improve the pressure characteristics of the fan.

In a recent configuration of tubeaxial fans, the blades are mounted on a hub which wholly or partially surrounds the motor itself, thereby minimizing the overall axial length of the fan. Fans of this latter construction have many attractive features, amongst which are relatively small size and relatively good performance capability.

However, tubeaxial fans of conventional design when used at their maximum performance capability, exhibit an effect at higher pressures which limit the range of application of any particular fan configuration. Specifically, in the pressure vs. flow characteristic of a tubeaxial fan, there occurs a region of instability called the centrax. In the centrax region, theperformance of the fan is so degraded (for example, there is a large loss in air flow as well as a loss in efficiency) that it becomes materially less eifective for its intended purpose. Consequently, where it is desired to achieve maximum performance capability, it is necessary to limit operation of the fan to a region of its characteristic beyond the centrax region.

As a result of this limitation on the tubeaxial fan, designers, when faced with a piece of equipment requiring substantial cooling flows at relatively high pressures, are forced to resort to larger fans or other forms of air moving devices. For example, vaneaxial fans or blowers of the squirrel cage or radial type, are capable of delivering substantial air flows against relatively high pres sures and are often used when a suitable tubeaxial fan is unavailable. By their nature, however, these devices take up more space or are considerably noisier than fans of the tubeaxial type.

Consequently, with fans of the types heretofore available, the equipment designer has been forced to make an appropriate selection from a large number of different size and type of fans to fit a particular application. To do so, it has been necessary to define precisely all of the pertinent performance and size criteria and strike the optimum balance. As a result, a great deal of design effort has been expended in fan selection, and fan manufacturers have been required to stock a large number of fans of varying sizes and types to accommodate the wide range of possible applications.

The primary object of the present invention is to provide a novel form of fan having performance characteristics spanning a range formerly accommodated by a number of different, separately designed units, whereby the equipment designers task is greatly simplified and the number of individual types of fans produced and stocked by manufacturers may be significantly reduced in number.

It is a further object of the present invention to provide a new type of tubeaxial fan which operates at good efiiciency over a substantially wider performance range than prior art fans of the same class.

Yet another object of the present invention is to provide such an improved tubeaxial fan in which greater performance capability is obtained in a smaller package than comparable fans of conventional form.

A further object of the present invention is to provide such an improved tubeaxial fan in which the greater performance capability is achieved with less power consumption and noise than comparable prior art fans.

Yet another object of the invention is to provide a novel type of tubeaxial fan having a substantially broader range of application by virtue of substantial elimination of the instability or centrax region in its pressure vs. flow characteristic.

As indicated above, the selection of a fan to suit a given application involves a balance of a multiutde of individual factors. The fans performance capability, i.e., its ability to generate an air flow against pressure, in conjunction with its blade diameter, overall size, power consumption, noise generation, and range of satisfactory performance must be taken into account. In accordance with the present invention, a fan design is provided which exhibits significant improvement over prior art devices in all of the foregoing characteristics. Thus, a fan of the type' disclosed herein having the same size blade, the same overall size, and consuming the same amount of power as a conventional fan of similar class, will provide a greater flow of air against a given pressure and do so while generating less noise. In addition, the fan of the present invention will be substantially more efiicient over a wider range of performance than conventional fans of the same class.

The improved overall performance of the fan of the present invention is evidenced by the virtual elimination of the centrax region of the fans operating characteristic. Since no instability occurs over the entire operating range of the fan, any given fan according to the invention can be employed over a correspondingly wider range of applications. Thus, the manufacturer can decrease the number of fan types that he must produce and conversely, the equipment designer need consider a fewer number of fans in selecting one forhis particular application.

Essentially, the significantly improved operating advantages of the present invention are achieved by developing and enhancing the free propagation of a radial velocity component at the output side of an axial flow fan. Structurally, this is accomplished by two means. Firstly, the fan blades and the air flow passage within the shroud are designed to provide a strong radial component of air flow in addition to the axial component. Secondly, an annular zone extending for a given distance radially outwardly and downstream of the tips of the fan blades is left completely unobstructed to allow unrestricted movement of the air flow having the strong radial component of momentum.

In broadest terms, the tubeaxial fan design of this invention is in combination of (1) a propeller designed to provide a substantial radially outward component of momentum to the air being propelled through the flow passage in which the propeller is mounted, and (2) a substantially unobstructed annular zone at the outlet of the fan, which unobstructed zone extends radially outward from the outlet rim of the shroud as well as downstream from the outlet end of the flow. passage.

The unobstructed zone, which is one of the essential features for fan design embodying the teachings of this invention, is at times herein termed a free vortex area. This terminology is employed because it is believed that the air being impelled downstream and out of the flow passage of the fan leaves the trailing tip of each fan blade with a flow pattern roughly similar to that of a free vortex and that it is important to the stable, quiet and efficient operation of the fan that this free vortex pattern be unobstructed as the air proceeds downstream from the fan. In order to obtain this free vortex area it is important that the fan flow passage terminate substantially no further downstream than the downstream tip of the. fan blades. Similarly, it is desirable that the spokes which support the hub on the housing be so located as not to obstruct the free vortex pattern, for example, at the upstream (inlet) side of the fan.

Specifically, it has been found, by smoke tests, high speed photography and other visual and quantitive means, that the unobstructed zone must start no further downstream than the trailing tips of the fan blades and that there must be no projections, spokes, housings or protuberances within the area described by the rotation of a line expressed by the exponential function:

W=Cr

Where:

w represents the axial or downstream spacing from the trailing blade tips r represents the radial spacing from the trailing blade tips n represents the exponent of r (greater than 2), and

represents a constant for the particular fan design and value of n. 3

This expression states mathematically the important fact that the most critical part of said zone is that nearest the blade tips and that such a zone contain a sharp step. Ob-

structions further and further downstream have a decreasing efiect as regards radial spacing. This relationship will hereinafter be discussed in greater detail.

In a preferred design, a flow passage that diverges in a downstream direction is employed. A diverging flow passage can be defined by a diverging inner wall of the shroud or, preferably, by such a diverging inner wall in combinaiton with a diverging hub.

Other objects and purposes of this invention will become apparent from a consideration of the following detailed description taken in connection with the accompanying drawings, in which: I

FIG. 1 is an elevation view of the outlet'end of a preferred embodiment of a fan of this invention;

FIG. 2 is a radial cross-section in partial relief along the plane 2-2 of FIG. 1, while FIG. 2A is a portion of a similar radial cross-section (taken at the plane 2A-2A in FIG. 1) near the trailing tip of the blade;

FIG. 3 is a plan view of the face of the fan blade employed in the FIG. 1 fan;

FIG. 4 is a section along the plane 4-4 of FIG. 3;

FIG. 5 is a section along 5-5 of FIG. 3;

FIG. 6 is a side elevation of FIG. 3;

FIG. 7 is :a radial cross-section in partial relief similar to FIG. 2 showing a means for mounting a fan designed in accordance with the teaching of this invention;

FIG. 8 is a radial cross-section in partial relief of a second embodiment of this invention in which the tapered hub is replaced by a straight hub;

FIG. 9 is a radial cross-section in partial relief of a third embodiment of this invention employing a straight flow passage;

FIG. 10 illustrates the kind of improvement that is obtained when a fan having a given size and power is redesigned in accordance with the teachings of this invention;

FIG. 11 illustrates the relative performance characteristics that are obtained when a typical prior art fan and a typical fan of this invention are each selected to have the same initial operating point P Q FIG. 12 represents the static efliciency curves that are typical of tubeaxialfans;

FIG. 13 illustrates two operating characteristic curves obtained from data taken on a prior art fan and the same size fan designed in accordance with the FIG. 8 embodi ment;

FIG. 14 illustrates two operating characteristic curves obtained from data taken on a fan with highly loaded blades designed in accordance with the FIG. 1 embodiment and a prior art fan with relatively lightly loaded blades;

FIG. 15 is also obtained from experimental data and contrasts the efficiency of the fan designed in accordance with the FIG. 2 embodiment and a typical prior art fan; and

FIG. 16 illustrates and contrasts the operating characteristics of the three embodiments of this invention shown in FIGS. 2, 8 and 9.

FIGS. 1 and 2 illustrates the preferred form of a fan 50 embodying this invention. In the embodiment illustrated, five fan blades 55 are mounted on a hub 52, the fan blades 55 are located in a flow passage 51, which flow passage is defined by the hub 52 and the inner wall 53 of the shroud 54. It should be kept in mind that this invention relates to a fan with blades within a flow passage 51 and not to a fan which operates in free air. The hub 52, as well as the blades 55, are supported within the flow passage by spokes 56 which extend out to the shroud 54. It might be noted at this point that the spokes 56 are located at the inlet side of the flow passage 51 so that the outlet side of the flow passage 51 is left unobstructed.

An electric motor is mounted inside the hub 52 to provide the power for rotating the hub 52 and fan blades 55. The motor is not illustrated or described herein since its operation and construction is known to the art. A more complete description of a motor and related mechanism may be found in Patent No. 2,926,838, issued to J. C. Van Rijn on Mar. 1, 1960.

There are two features of a fan designed in accordance with the teachings of this invention which are of particular significance. These features are the shape of the fan blade and the nature of the area at the outlet of the flow passage.

Fan blades One of the crucial features to the proper operation of the invention is in the design of the fan blades 55. Accordingly, a fan blade design is shown in FIGS. 3 through 6 which is an example of a type of fan blade that must be employed in any fan that is an embodiment of this invention.

Important aspects of the shape of the preferred fan blade 55 to be used in the fan 50 of this invention are described in my Patent No. 1,964,525 issued on June 26, 1934. Patent No. 1,964,525 teaches a blade shape which counteracts the inward momentum of air in free air fans or fans operated in large radii orifices. When the fan has a flow passage 51 as in the fan 50 of this invention, the shroud 54 serves to restrict the inward movement of the air. Accordingly, the blades of the type described in Patent No. 1,964,525, when incorporated into a fan having a flow passage (such as the flow passage 51 shown herein) serve to move the air in a diverging manner rather than serving to counteract the converging movement of the air. Therefore, the blades of the type described in Patent No. 1,964,525 are incorporated into the fan 50 of this invention for a purpose different than the purpose described in Patent No. 1,964,525. Thus, there may be some value in summarizing the structure of the fan blades 55 and the pertinent teaching of Patent No. 1,964,525.

The shape of the blades 55 can best be understood by reference to FIGS. 3, 4, 5 and 6. The operating or front surface 61 of the blade 55 is generally concave transversely; that is as one moves from leading edge 63 to trailing edge 65, one follows a concave surface as is seen by the sections shown in FIG. 4 and FIG. 5. The portion 66 of the concave front surface 61 near the trailing edge 65 has a small radius of curvature and thus exhibits a sharp curve terminating in the trailing edge 65. Thus the portion 66 of the blade near the trailing edge 65 extends forwardly from the face 61 of the blade in the direction of air flow. This forwardly extending portion 66 gives the final impetus to the air to move it away from the face 61 of the blade 65. The forwardly extending portion 66 in conjunction with the shape of the trailing edge 65 produces a component of force by its action on the air to provide an outward momentum to the air and thus propel the air through the flow passage 51 in a generally diverging manner.

The trailing edge 65 is rearwardly inclined over most of its length. In particular near the outer portion of the fan blade, the trailing edge 65 becomes sharply inclined rearwardly (30 is a frequently desirable rearward angle for the outer portion 65y of the trailing edge 65).

The outer portion 67 of the blade 55 is generally rearwardly extending from the leading edge 63 to the trailing edge 65. The rearward angle of this rearwardly extending The outer tip 68 of the blade 55 is cut to conform to the inner surface 53 of the shroud 54. In the embodiment shown in FIGS. 1 and 2, this inner surface 53 is diverging and thus frusto-conical. FIG. 2 shows the blade 55 in relief as it would actually be seen if a radial cut were taken through the shroud 54 at the mid point of the blade 55. The section 2-2 is taken at approximately the axial mid point of the blade 55 so that the blade 55 in FIG. 2 is seen as being close to the surface 53 only at its mid point. FIG. 2A is shown to clarify the relationship between the outer tip 68 of the blade 55 and the inner wall 53 of the shroud 54. FIG. 2A is a portion of a radial cross-section of the shroud 54 at a position near the trailing tip 65y of the fan blade 55. Thus FIG. 2A

shows the relationship between the trailing tip 65y of the fan blade 55 and the inner wall 53 of the shroud 54. With reference to FIG. 2, rotation of the upper fan blade 55 into the paper would bring the trailing tip portion 65y into close spaced relationship with the portion of the inner sidewall 53 shown. The point here is that the outer edge 68 of the blade is uniformly close to the sidewall 53 surface over most of the length of the edge 68 right up to the trailing tip 65y (which trailing tip 65y is the portion of the blade 55 closest to the fan outlet).

As is pre-eminently true in the case of the design of fans, each particular fan will require some experimentation to obtain the preferred details as to dimensions and extent of radial thrust imparted to the air by the fan blades. However, such variations are usual in this art and it will be understood that one skilled in the art knows that the design of each fan must be accompanied by some experimentation as to these and the many other factors which go into fan blade design.

Free vortex zone The second critical limitation that must be observed in connection with any fan embodying this invention is that a zone is left completely unobstructed at the outlet of the flow passage 51. This unobstructed zone must extend both downstream and radially outward from the flow passage 51 outlet. More specifically, what is required is that the air which leaves the outer portion 65y of the trailing edge 65 of the fan blade 55 must be left free to follow the flow pattern developed by the type of fan blades 55 described above which are employed in the fan 50 of this invention. It is believed that the pattern of flow can be analogized to a free vortex type of flow, or at least such an analogy can be made in the zone immediately downstream from the trailing edge of the fan blades 55. Thus the so-called free vortex zone must begin at the outer portion 65y of the trailing edge 65 of the fan blade 55. Accordingly, it becomes essential that the shroud 54 terminate (in the downstream direction) at the trailing tip 65y of the fan blades 55. Where safety conditions permit, it may even be desirable for the shroud 54 to terminate a fraction of an inch before the trailing tip 65y of the fan blades 55. In any case, it is clear from experiments that have been run that any appreciable exouter portion 67 increases as one travels along the rearwardly extending portion 67 from the leading edge 63 to the trailing edge 65. The net efl ect of the rearwardly extending outer portion 67 is to give an outward component of momentum to the air and to cause the air to travel in a diverging fashion through the diverging flow passage 51. The rearwardly extending outer portion 67 may extend over about one-third of the radial extent of the blade surface 61. Thus, in general terms, the front surface 61 of the blade 55 will appear convex in a radial direction while it appears generally concave in an axial or transverse direction.

tension of the shroud 54 in a downstream direction will obviate the advantages obtained by this invention.

It has generally been understood in the design of simple propeller tubeaxial fans that optimum performance and minimum turbulence is obtained by placing the spokes which support the hub in the shroud at the outlet end of the fan. For example, see the design of the fan shown in Patent No. 2,926,838, issued to]. C. Van Rijn on Mar. 1, 1960. However, in the particular embodiments of this invention illustrated in FIGURES 1, 2, 7, 8 and 9, in order to render the free vortex zone totally uninterrupted by structural members which would serve to disturb the free vortex air flow, and thus enable maximum realization of the advantages of the present invention, the spokes 56 that support the hub 52 are placed at the inlet to the flow passage 51 rather than at the outlet.

FIG. 7 illustrates a radial section similar to FIG. 2 in which the housing 54a is designed to include a radially outward setback at the outlet of the flow passage 51 so as to provide an annular mounting ring 69. The advantage of building in the annular mounting ring 69 is so that in use the fan designed in accordance with the teaching of this invention will be mounted in such a fashion as to assure that the free vortex Zone is maintained.

The vortex spirals 72 and 73 shown in FIG. 7 are simply a schematic illustration of what is believed to be the dominant mode of air travel after it leaves the trailing tip 65y of the blade 55. In FIG. 7, the spiral 72 illustrates the path of air leaving the trailing tip of the upper fan blade 55 while the spiral 73 illustrates the path of air leaving the trailing tip of the preceding fan blade. Because the air as it leaves the fan blades 55 has a component of momentum in the direction of travel of the fan blades, the spirals 72 and 73 curve around the flow passage 51 outlet as well as spiraling and progressing downstream. The complex nature of the travel of air emitted from the flow passage 51 is caused by the fact that there are components of momentum imparted to the air in all three coordinates. The fan blades 55, as discussed above, are shaped to provide a substantially radially outward component of momentum as well as a downstream component of momentum. The fact that the fan blades are rotating further imparts a component of momentum in ,the direction of rotation.

The spiral fiow paths 72 and 73 illustrated are only a rough illustration of what actually occurs. The spirals will vary from fan to fan in such characteristics as the diameter of the spiral, the distance between successive spirals from successive fan blade tips and the extent to which the spirals will extend downstream before breaking up. Even where a single fan is considered, a change in the operating conditions of the fan will change the characteristics of the spiral flow. For example, when a larger quantity of air is being circulated, the faster flow of air downstream from the fan will cause the successive spirals to be further apart longitudinally than when a smaller amount of air is being circulated.

The dash-dot lines shown in FIG. 7 and indicated by the formula w=Cr represents the envelope within which the hypothesized vortex flow occurs. This formula is a very generic type and relates the radial setback r to the length w of the ring 69 in a design such as FIG. 7 where an annular ring is employed. Assuming that the fan will be connected to an opening in a frame that leads into an open interior and not into a duct, then it can be seen that the radial setback r may be reduced as the longitudinal length w of the annular ring 69 is reduced. In one fan having a 2% inch radius to the outer tip of the fan blade, a magnitude for the constant C of 9.4 and for the exponent n of approximately 3 seems to about define this envelope. Because the radial setback r increases as a fairly high order root of the length w of the ring 69, it becomes possible to mount the fan of this invention on a duct if the annular setback ring 69 is made long enough and set back sufficiently far. There is every reason to believe that the constants C and n are a function not only of the particular fan but also of its operating conditions.

Other features In the preferred form illustrated in FIGS. 1 and 2, the hub 52 and innerwall 53 of the shroud 54 diverge over most of their length. In this application, the characarea along the length of the flow passage 51 as is possible.

Thus the angle of divergence of the hub 52 will be a few degrees greater than the angle of divergence of the sidewall 53.

FIG. 8 is a radial cross-section in partial relief similar to the cross-section shown in FIG. 2 with the exception that the FIG. 8 embodiment has a straight hub 52a rather than the diverging hub 52 of the FIG. 2 embodiment. The essential nature of the fan blades 55a is the same as that described in connection with the FIG. 1 embodiment, the only difference being that the radially inner portion of the fan blades 55a is adapted to fit on the straight hub 52a instead of the diverging hub 52.

FIG. 9 illustrates a third embodiment by means of a radial cross-section in partial relief similar to the FIG. 2 and FIG. 8 illustrations. The FIG. 9 embodiment employs a shroud 54b which has a straight innerwall 53b as well as a straight hub 52a so that the flow passage 51!) is a straight, as contrasted with diverging, flow passage.

The fan blades 55b in the FIG. 9 embodiment are essentially similar to the fan blades 55 discussed and shown in detail in connection with the FIG. 1 embodiment except that the leading portions of the outer edge 68 are extended radially outward to conform with the straight sidewall 53b.

The relative significance of the embodiments shown in FIGS. 2, 8 and 9 can best be understood by an inspection of the performance characteristic curves shown in FIG. 16 and will be discussed in detail further on.

Performance characteristics The curve 70 in FIG. 10 represents the pressure-flow operating characteristic for a conventional tubeaxial fan. A pronounced centrax area 70c is shown. A fan which is normally designed to operate substantially to the right of the centrax area may be employed in an application where back pressure builds up. As back pressure does build up, the operating point of the fan will ride up the curve 70 until it enters into the centrax area. The entry into the centrax area 70!: might be deemed a stall point since great instability and noise and sometimes stalling of the fan occurs at such a point. However, it is not only the noise and instability, as undesirable as those characteristics are, which detract from the performance of the fan in the centrax area 700. The leveling out of the pressure-flow characteristics 70 at the centrax area 700 also means that the volume of air which the fan is able to provide decreases markedly at pressures that bring operation into the centrax area.

The nature and magnitude of the improvement in fan performance elfected by this invention may in part be appreciated by comparison of the two idealized curves 70 and 72 in FIG. 10. Curve 72 represents the operating characteristic of a fan designed in accordance with the FIG. 1 embodiment of this invention while curve 70 represents a typical conventional tu'beaxial fan. These two curves represent fans which are identical in all respects such as size, power, number of fan blades, etc., except for those features explicitly taught by this invention. In particular, the fan represented by curve 72'ditfers from the fan represented by curve 70 in that the former has: (1) a blade adapted to develop a significant radially outward component of momentum to the air being driven through the flow passage, (2) a free vortex zone at the outlet of the fan, and (3) a diverging sidewall in conjunction with a diverging hub to define the flow passage. A comparison of curves 70 and 72 shows that the fan of this invention provides a greater volume of air at all static pressures, and, because the centrax area is eliminated, circulates a very much greater volume of air at the higher pressures.

Because of the overall improvement in fan performance, as shown by FIG. 10, the equipment designer is able to select a fan designed in accordance with this invention that is somewhat smaller than the conventional tubeaxial fan in order to develop any desired air flow at a given pressure. Thus, with reference to the idealized curves in FIG. 11, if the equipment designer wishes to desig-n for the initial operating point P Q he may select either a conventional tubeaxial fan (and thus obtain the operating characteristic curve 74) or a fan of this invention (and thus obtain the operating characteristic curve 76). Be cause of overall improved performance (see FIG. 10), the fan of this invention that performs at point P Q will be smaller in size and require less power than does a conventional fan that operates at point P Q The employment of the fan of this invention not only permits the use of a smaller fan but, as may be seen from a comparison of curves 74 and 76, provides a much more desirable type of operating characteristic in that the centrax area is eliminated.

In fan applications where pressure builds up over a period of time (as where the air that is being circulated is drawn through a dust filter), there will be a very rapid drop off in the cooling air available, if a conventional fan is used, once the pressure has risen to the point that causes the fan to operate in the centrax area 74c. However, if the fan of this invention is employed, the drop off in air flow as pressure increases is much steadier and smaller. This results in a much more effective cooling action as pressure builds up and generally also means a more efiicient and less noisy operation.

In FIG. 12, curves 78 and 80 represent the static efiiciency curves that are typical respectively for the conventional fan having the characteristic curve 74 and for the fan of this invention having the characteristic curve 76. The improvement in efliciency is greatest in the area of the characteristic where the conventional fan exhibits centrax but there is improvement in efficiency at all or substantially all operating points when a fan designed in accordance with this invention is employed in lieu of a conventional tubeaxial fan. This improvement of efiiciency exemplified by curve 80 as contrasted with curve 78 comes about in part because of the elimination of the very ineflicient operation at centrax. The fan designed in accordance with this invention and having the operating characteristic curve 76 will be smaller in size and require a less powerful motor than does the conventional tubeaxial fan that would have the operating characteristic shown as curve 74 in FIG. 11.

The fans that are represented by the operating characteristic curves 74 and 76 in FIG. 11 are both fans that are highly loaded. This means that both of the fans have their blades designed to provide maximum or near maximum air flow. It might be possible to duplicate or nearly duplicate the desirable operating characteristic curve 76 with a conventional tubeaxial fan by selecting a larger tubeaxial fan and unloading the fan by redesigning the blades. The static efliciency curve 82 in FIG. 12 is an example of the efiiciency curve of such a conventional fan that is selected large enough and has been unloaded enough to provide an operating characteristic curve similar to the curve 76. The loss of efficiency and/r increase in size however are material and make it unfeasible to employ this approach to the design and use of fans.

Thus it may be seen that the great advantage of the fan of this invention is that it provides superior performance in a smaller package with a saving of power. It is this combination of advantages which makes the fan of this invention an overall improvement in tubeaxial fans.

In addition to the fact that the fan design of this invention permits an equipment designer to use a smaller and more efficient fan for a given pressure-flow condition, the elimination of centrax makes it feasible to operate a fan over a broader area of its operating characteristic curve. The fan having the operating curve 76 of FIG. 11 and efiiciency curve 80 of FIG. 12 can be efiiciently and efiectively employed over a wide range of pressures. From a fan manufacturers point of view, this means that many 10 to select an effective and efficient fan for a given operation.

FIGS. 13 through 16 illustrate curves prepared from performance measurements on conventional fans and fans built in accordance with this invention.

With reference to FIG. 13, a conventional tubeaxial fan, having the operating characteristic curve 84, was taken and converted to a fan of the FIG. 8 embodiment. All that was done to the conventional fan was to replace the fan blades and to employ a shroud having a diverging sidewall. No other changes were made. The converted fan provided the operating characteristic curve 86. Of course, the converted fan was operated so as to impose no obstructions in the free vortex zone at the outlet of the flow passage. The curve 86 shows a vestige of a centrax. Other tests show that even this vestige will be eliminated by the use of a diverging hub. In any case, the performance represented by curve 86 is in all senses superior to that represented by curve 84. At a pressure of 0.18 inch of Water, the conventional fan circulated 40 cubic feet per minute (c.f.m.) of air while the fan of this invention circulated about 72 c.f.rn. of air. At most pressures the improvement was not that startling but there is substantial improvement all along the line.

As discussed above, it is possible to eliminate or minimize centrax by unloading a fan. In FIG. 14, curve 88 represents what happens when a conventional fan is redesigned by having the pitch of its fan blades decreased. Curve 90 is the operating characteristic for a comparable fan of the FIG. 1 embodiment (having diverging hub and sidewall). The fans represented by curves 88 and 90 are similar in size and power requirements. Thus it may be seen that in order to eliminate centrax with a conventional type of fan, a great loss in performance must be accepted.

FIG. 15 shows little more than is illustrated by FIG. 12. However, FIG. 15 is taken from data and thus gives an actual picture of the improved static efliciency obtained in fans of this invention. The fans represented by curves 92 and 94 were both highly loaded. The fan represented by curve 94 was of the FIG. 1 embodiment, having diverging hub and sidewall. The point underlined by FIG. 15 is that the improvement obtained by this invention is reflected in all the significant characteristics of the fan. Centrax is eliminated; the operating characteristic is improved over its entire length; and size and power requirements are reduced.

FIG. 16 provides a comparison of the various embodiments of this invention. As may beseen from curve 96, the centrax is substantially eliminated by the basic combination of (1) a blade designed to impart a significant radially outward component of momentum to the air flowing through the flow passage of an axial propeller fan, and (2) a free vortex area immediately downstream of the fan blade. This is the FIG. 9 embodiment. Curve 98, reflecting the FIG. 8 embodiment, shows that a further improvement is obtained by adding to the above basic combination the feature of a flow passage with a sidewall which diverges in a downstream direction. The greatest improvement is obtained with the FIG. 2 embodiment, represented by curve 100, where the flow passage is defined by a diverging hub in combination with a diverging sidewall.

Curce 96 illustrates the extent to which an improvement in pressure-flow characteristic may be obtained in a fan of this invention even though the flow passage is straight.

The curve 98 is a pressure-flow characteristic for a fan of the type illustrated in FIG. 8. The curve 98 thus represents a fan with a tapered sidewall, but a straight hub.

' As may be seen from a comparison of the curves 96 and fewer fans are required to provide a complete line of fans.

From the fan nsers point of view, it becomes much easier 98, the addition of a tapered sidewall introduces a slight improvement in the pressure-flow characteristic at the higher back pressures. Thus at a pressure of 0.20 inch of water, the tapered sidewall fan of FIG. 8 (represented by the curve 98) provides a flow of about 24 cubic feet per minute while the straight flow passage fan of FIG. 9 (represented by the curve 96) provides a flow of .only 22 cubic feet per minute.

The curve 100 represents the same basic fan as is represented by the curves 96 and 98 except that a tapered hub has been added. With the tapered hub, the flow rate at the higher static pressures is increased and the centraX area is completely eliminated. Curve 100 represents the kind of pressure-flow characteristic obtainable with the fan design illustrated in FIGS. 1 through 6.

This invention has been described in connection with a tubeaxial fan because it has no application to the design of free air fans or to centrifugal fans. However, experience has shown that it is possible to place appropriately designed pre-twisters at the inlet of a tubeaxial fan designed in accordance with the teachings of this invention. Accordingly, it should be understood herein that the claims are directed to a tubeaxial fan with or without whatever additional features may be desired in certain circumstances such as the pre-twister feature.

It should also be understood that a fan designed in accordance with the teachings of this invention can be connected to air ducts provided the appropriate free vortex zone is incorporated in the system. The type of frame design shown in FIG. 7 would make it possible to directly connect the outlet of the fan to a duct since the FIG. 7 frame would assure the maintenance of a free vortex peripheral zone.

What is claimed is:

1. In a tubeaxial fan adapted to develop pressure, said fan having a flow passage defined by a hub and a sidewall with fan blades within said flow passage mounted on said hub, the improvement comprising:

(a) means in said flow passage to impart a substantial radially outward component of momentum to the air being driven through said flow passage by said fan blades, and

(b) a substantially unobstructed peripheral annular zone at the outlet of said flow passage, said zone extending sufiicient distances downstream and radially outward from the downstream termination of said sidewall to enable the air leaving said flow passage to follow freely the flow patterns developed in its traversel of said passage, said downstream termination of said sidewall being substantially no further downstream than the downstream tip of said fan blades.

2. In a tubeaxial fan adapted to develop pressure having a hub and a sidewall defining a flow passage therebetween, the improvement comprising:

(a) fan blades in said flow passage to impart a substantial radially outward component of momentum to the air being driven through said flow passage by said fan blades, and

(b) a substantially unobstructed peripheral annular zone at the outlet of said flow passage, said zone extending sufficient distances downstream and radially outward from the downstream termination of said sidewall to enable the air leaving said flow passage to follow freely the flow patterns developed in its traversal of said passages, said downstream termination of said sidewall being substantially no further downstream than the downstream tip of said fan ,blades.

3. The fan of claim 2 further characterized by a mounting ring extending downstream from said outlet of said fan, said mounting ring being spaced radially outward of said downstream termination of said sidewall by said suflicient distance.

4. The fan of claim 2 further characterized by said sidewall diverging in a downstream direction.

5. The fan of claim 4 further characterized by said hub diverging in a downstream direction to provide a flow passage having both innerwall and outerwall diverging.

6. The fan of claim 5 wherein the cross-sectional area of said flow passage between said diverging hub and said diverging sidewall. is substatnially constant at all axial positions.

7. The fan of claim 5 wherein said innerwall has a substantially constant angle of divergence, said angle of divergence being between 10 and 20.

8. The fan of claim 5 further characterized by a mounting ring extending downstream from said outlet of said fan, said mounting ring being spaced radially outward of said downstream termination of said sidewall by said sufficient distance.

9. An axial flow propeller fan adapted to develop pressure comprising:

(a) a diverging hub having a substantially constant angle of divergence and terminating in a diverging mode at the outlet of said fan,

(b) a peripheral shroud having a diverging innerwall terminating in a diverging mode at the outlet of said fan, said innerwall having a substantially constant angle of divergence, said angle of divergence being between 10 and 20,

(c) said hub and said innerwall defining a flow passage, the angle of divergence of said hub and the angle of divergence of said flow passage having a relationship to one another such that said flow passage has a substantially constant cross-sectional area along most of its length,

((1) fan blades in said flow passage to impart a substantial radially outward component of momentum to the air being driven through said diverging flow passage by said fan blades, and

(e) a substantially unobstructed peripheral annular zone at the outlet of said fan, said zone extending sufficient distances downstream and radially outward from the downstream termination of said sidewall to enable the air leaving said flow passage to follow freely the flow patterns developed in its traversal of said passage, said downstream termination of said sidewall being substantially no further downstream than the downstream tip of said fan blades.

10. In a tubeaxial fan adapted to develop pressure, said fan having a flow passage defined by a hub and a sidewall with fan blades within said flow passage mounted on said hub, the method of circulating air comprising the steps of:

(a) developing a vortex flow pattern in the air emitted at the outlet of said flow passage by imparting a substantial radially outward component of momentum to the air being driven through said flow passage by said fan blades, and

(b) permitting the free development of said vortex flow in the area downstream and radially outward from the outlet of said flow passage.

11. A fan comprising a motor-containing hub having coaxial stationary and rotating portions, a plurality of fan blades fixed to the peripheral surface of said rotating'portion to be rotated therewith, a shroud surrounding said hub and fan blades and concentric therewith to define a generally annular flow passage, a plurality of support members extending from said stationary portion of said hub to said shroud to retain the latter in concentric relation to said hub, said fan blades being shaped to establish fluid flow through. said flow passage with said stationary portion and said support members at the upstream end and said fan blades at the downstream end, said shroud and said fan blades terminating at the downstream end of said flow passage in substantially the same plane perpendicular to the hub axis, and a substantially unobstructed peripheral annular zone at the outlet of said flow passage, said Zone extending sufiicient distances downstream and radially outward from the downstream end of said shroud to enable the fluid leaving said flow passage to follow freely the flow patterns developed in its traversal of said passage.

12. A fan according to claim 11 wherein the interior surface of said shroud surrounding said fan blades diverges from said axis in a downstream direction and the outer edges of said fan blades conform closely to said diverging surface.

13. A fan according to claim 12 wherein the peripheral surface of the rotating portion of said hub diverges from said axis in a downstream direction to define with the interior surface of said shroud a divergent flow passage.

14. A fan according to claim 13 wherein said fan blades are formed to impart a substantial radially outward component of momentur to the fluid driven thereby through said flow passage.

References Cited UNITED STATES PATENTS FOREIGN PATENTS 8/ 1949 Australia.

ROBERT M. WALKER, Primary Examiner. H. F. RADUAZO, Assistant Examiner. 

1. IN A TUBEAXIAL FAN ADAPTED TO DEVELOPE PRESSURE, SAID FAN HAVING A FLOW PASSAGE DEFINED BY A HUB AND A SIDEWALL WITH FAN BLADES WITHIN SAID FLOW PASSAGE MOUNTED ON SAID HUB, THE IMPROVEMENT COMPRISING: (A) MEANS IN SAID FLOW PASSAGE TO IMPART A SUBSTANTIAL RADIALLY OUTWARD COMPONENT OF MOMENTUM TO THE AIR BEING DRIVEN THROUGH SAID FLOW PASSAGE BY SAID FAN BLADES AND (B) A SUBSTANTIALLY UNONSTRUCTED PERIPHERAL ANNULAR ZONE AT THE OUTLET OF SAID FLOW PASSAGE, SAID ZONE EXTENDING SUFFICIENT DISTANCES DOWNSTREAM AND RADIALLY OUTWARD FROM THE DOWNSTREAM TERMINATION OF SAID SIDEWALL TO ENABLE THE AIR LEAVING SAID FLOW PASSAGE TO FOLLOW FREELY THE FLOW PATTERNS DEVELOPED IN ITS TRAVERSEL OF SAID PASSAGE, SAID DOWNSTREAM TERMINATION OF SAID SIDEWALL BEING SUBSTANTIALLY NO FURTHER DOWNSTREAM THAN THE DOWNSTREAM TIP OF SAID FAN BLADES. 